Process of preparing bromine heptafluoride

ABSTRACT

1. A process for the preparation of bromine heptafluoride comprising the steps of reacting bromine pentafluoride with fluorine in the presence of a compound having the formula MBrF6, wherein M is an alkali metal, at a temperature of from about 110* C. to about 340* C. and separating bromine heptafluoride from the unreacted reactants and other reaction products.

United States Patent [72] Inventors CharlesE.Fogle Sunnyvale; Robert T. Rewick, Mountain View, both of Calif. [21] Appl. No. 457,901 [22] Filed Apr. 28, 1965 [45] Patented Oct. 26, 1-971 [73] Assignee United Aircraft Corporation East Hartford, Conn.

[54] PROCESS OF PREPARING BROMINE HEPTAFLUORIDE 9 Claims, 2 Drawing Figs.

[52] U.S. Cl. 23/205, 149/] 149/36 [51] Int. Cl C06b 7/24 [50] Field of Search 23/205 [56] References Cited OTHER REFERENCES Schumb et al.. lndustr. Engng. Chem.. Vol 42, (1950) pp. 1383 to 1386TP1A58.

Emeleus et al., Advances in Inorganic Chemistry and Radiology, Vol. 3, 1961, p. 139, Qd 1A35.

Primary Examiner Leland A. Sebastian Attorney-Steven F. Stone CLAIM: l. A process for the preparation of bromine heptafluoride comprising the steps of reacting bromine pentafluoride with fluorine in the presence ofa compound having the formula MBrF wherein M is an alkali metal, at a temperature of from about 1 10 C. to about 340 C. and separating bromine heptafluoride from the unreacted reactants and other reaction products.

PATENTED GET 2 6 WI CHARLES E. FOGLE ATTORNEY PROCESS OF PREPARING BROMINE HEPTAFLUORIDE This invention relates to complex fluorides and more particularly to bromine heptafluoride and methods for production thereof.

Bromine heptafluoride (BrF-,) is a very reactive oxidizing agent, has a density comparable to that of bromine pentafluoride and has a fluorine content and bond energies equivalent to chlorine trifluoride. Although a gas at room temperature, it is readily condensable to a storable liquid. When used as the oxidizer in a liquid fueled bipropellant system using hydrazine as a fuel, a theoretical density impulse of 500 sec. is realized. This represents an increase in performance of about 40 percent over state of the art bipropellant systems such as LOX-Kerosene, and an improvement of about percent over'the most advanced systems now known.

However, prior to this invention of BrF, and the method for preparing it, the compound itself was unknown and was con sidered to be incapable of synthesis. As was stated in The Journal of the American Rocket Society, Vol. 29, No. 2, in an article entitled Recent Advances in Fluorine Chemistry and Technology" under the subhead Halogen Fluorides and Complex Fluoridesz Since 1931, when bromine pentafluoride was first prepared 35, no new halogen fluorides have been found, and one of these previously accepted, bromine monofluoride, can probably not be isolated 36. It would be an achievement to prepare, for example, ClF, or BrF however, stereo chemistry suggests that these will not be made."

Support for this position appears in the fact that the only known halogen heptahalide is iodine heptafluoride (See Comprehensive Inorganic Chemistry, Vol. 3, Sneed et al., Van Nostrand (1957) P. 231.

Contrary to the teachings of the prior art, we have developed a method for preparing BrF, and have isolated the compound.

Accordingly, the primary object of this invention is the new compound, bromine heptafluoride.

Another object of this invention is a method for preparing, recovering and purifying bromine heptafluoride. These and other objects of the invention will be readily apparent from the following description with reference to the drawings wherein:

FIG. 1 is a schematic representation of the apparatus used to form Brl-'-,, and

FIG. 2 is a schematic representation of the apparatus used to purify BrF,.

The reaction route is basically a fluorine addition reaction to BrF, according to the equation:

The presence in the reaction mixture of MBrF where M is Cs or K appears to facilitate the formation of BrF, according to the following equation:

The MBrF, may be added separately to the reaction mixture, but, preferably, it is formed in situ by adding MP to the reactants containing an excess of BrF The process may be practiced both on a static or a flow basis. The yields are relatively low, making the recovery of the product from the reactants fairly difficult.

It has been found that relatively pure BrF, can be recovered by condensation by passing the crude reactants through a series of cold traps cooled to 80 C. and 183 C. The semirefined condensate or crude is then purified by fractionation in a fractional codistillation apparatus.

Apparatus suitable for the synthesis of BrF, is shown in FIG. 1. The reactor 1 consisted of a nickel cylinder 6 in. in length by 1% in. in diameter with a capacity of 180 ml. to which was attached a pressure gauge 2 having a monel bourdon tube. Nitrogen, fluorine and bromine pentafluoride feed lines 3, 4 and 5 respectively are connected to a NaF scrubber 6 which in turn is connected through suitable valving V to vent line 11, cold trap 19 and soda-lime scrubber 20. In a typical run in which CsBrF is formed in situ from CsF and BrF 99.52 percent pure CsF with traces of other alkali metal fluorides is charged into the reactor 1 which is evacuated by pump 17 and the CsF dried at 150 C. for 16 hours. The system is returned to ambient pressure with pure nitrogen from cold trap 19. BrF, with a minimum purity of 98 percent is further purified by distillation into the reactor 1 through the Na! scrubber 6 maintained at 175 C. The fluorine, with a purity of 98+percent, is also further purified by passing it slowly through the NaF scrubber 6 and traps 7, 8 and 9, cooled with liquid oxygen, into holding tank 10. This process removes all impurities with the exception of traces of CF, and HF. A portion of the purified fluorine is then condensed into the reactor 1 by cooling with liquid nitrogen. Upon warming to room temperature, the reactor pressure increases to about 250 to 270 p.s.i. and pressures of from 500 to 4,750 p.s.i. are observed at reaction temperatures varying from C. to 340 C. The reactor can be agitated during the reaction period if desired. After a predetermined reaction time which varied from 4 to 128 hours, the reactor is allowed to cool to room temperature and pressure is released slowly through the fractionation system consisting of trap 7 cooled with dry ice-acetone to a temperature of 79.8 C. and traps 8 and 9 cooled to liquid oxygen temperature of 183 C. The great majority of the condensable gas collects in trap 7 and a small amount of material con denses in traps 8 and 9. When the system reaches 1 atm, it is flushed with gaseous nitrogen and then evacuated. The infrared spectrum of each fraction is obtained by removing the fraction from the system through infrared cell inlet 14. An unidentifiable infrared absorption peak was found at 735 cm-1 which corresponds to a theoretical prediction of an absorption peak for BrF, close to 735 cm-l. The crude material showing the 735 cm-1 peak was highly contaminated and was purified by means of the fractional codistillation apparatus shown in FIG. 2. s

In a typical separation, a crude sample is connected to the apparatus at inlet 21 and either frozen into transfer U-tube 22 or separation U-tube 23. If the sample is large, it is first frozen into 22 and then transferred with a flow of helium from source 24 and warming to separation U-tube 23. Dewar vessels 25 and 26 are provided around U-tubes 22 and 23 and after transfer is completed, transfer U-tube 23 is cooled to liquid nitrogen temperature and helium flow is adjusted to about 1 l0 cc./min. Dewar vessels 25 and 26 are quickly removed, emptied and replaced around the U-tubes. The arms of U-tube 23 are connected to channels in a monel block 28 which are sealed at the upper end and contain heating elements 29. Heum fed from inlet 30, cooled by passage through U-tube 22, cools block 28 by passage through Teflon tubing 31 which surrounds block 28. This gas flow helps to maintain thermal equilibrium and is bypassed around collection U-tube 32 until a temperature of C. is obtained as measured by thermocouple 33. At this time collection U-tube 32 is cooled to liquid nitrogen temperature by means of dewar vessel 34 and maintained at 1 atm pressure. At a temperature of l27 C., gas flow is directed through collection U-tube 32, flowmeter 35 and out exit line 36. When a temperature of l 10 C. is reached, gas flow is again diverted around collection U-tube 32. The material in tube 32 is subsequently vacuum transferred to measuring U-tube 37 by means of vacuum line 38 and can be removed from the system through takoff lines 38. Pressure gauges 40 and 41 are located as shown. The apparatus, with the exception of block 28 and Teflon tubing 31 was constructed of copper tubing with valve means V employed as shown. The cut delivering BrF, was determined at l27 C. to 1 10 C. by successive narrowing of the collection range and analysis of the material.

The above-described procedure is effective for separating small amounts of gas by carrier gas distillation whereby the vaporized sample is carried into small diameter columns, either packed or unpacked, maintained at liquid nitrogen temperatures. Because a temperature gradient exists in the cold column, the substances in the sample are fractionated and occupy zones within the column; the most volatile species being farthest from the inlet and the least volatile closest thereto. Slow warming of the column maintains a temperature gradient and each component is eluted from the column by the carrier gas in order of decreasing volatility. Flow can be directed allow the reaction of greater molar quantities of the gaseous reactants and are accordingly desirable.

Within the operating temperature range of 1 10 C. to 340 C., there appears to be no temperature dependence. However,

through a thermal conductivity cell connected to a recording 5 above about 250C. there appears to be some thermal decom potentiometer whereby peaks are obtained which represent Position Ofihe 1- thermal conductivity differences between the effluent and BrF hydrolyzes to yield bro n yg n r mi and pure carrier reference which are indicative of the concentrahydrofluoric acids. It is compatible with nickel monel, copper tions of the individual components. Emergence of th and stainless steel in decreasing order, and is at least partially separated substances in order of decreasing volatility also aids Stable at temperatures as g 35 A r mper ture in identification. Also, since the initial rise of a peak is due to it is a gas but is ily condensable. having a oiling point of each components initial vapor pressure, a correlation with approximately 00 melting point is possible. The specific examples of the process in the above desel'lptioni CSBrFs has n used as he set forth in table I show production of crude product. When preferred catalyst material. The process also has been conthe crude products were further refined by the above fracl5 ducted with hin One experiment, 6 moles of 2 n 4.3 tional codistillation ro a ou t f u B F varying moles KF were reacted with a stoichiometric excess of BrF of from less than 1 mg. to about 3 mg. were obtained. The pure 0.] 1 moles at a temperature of l80 C. for 230 minutes at atproduct was analyzed on the mass spectrometer and corn. mospheric pressure. A weak peak at the 735 cml infrared pared with the mass spectrometer readings on BrF as shown band Indicated the Presence of r- The use of other in table ll. The differences in peak height for the ion species Pounds of the formula r with M belng n li me al is BF+ through BrF +f0r BrF and BrF, are attributed to the lncontemplated herein with the higher atomic Weight metals m i diff i h b d strengths between and being more effective than the lower. The invention has been BrF-,. The presence of the peak at m/e 212 shows the presence described with respect to specific examples thereof, however, f rderived f 3 -17 The presence f a peak at 193 it should not be construed as limited thereto. The invention inalso shows BrF since the BrF ion could only be formed by cludes all modifications and substitutions which are within the the degradation ofa higher interhalogen. Scope of followmg Clams As noted above, table I indicates the amount of crude BrF, we Clalmi produced. The amount of crude, however, is not necessarily Process for the prepaliauon of Pmmme heplafluonde indicative of the amount of pure BrF produced. Instead, the comPrlslllg the Steps of reactmg bromme pehtafluoflde with relative height of the 735 cml infrared band provides the lflfluorine m e P of a Compound having the formula dication of the amount of BrF present before purification. As MBTFfii wilcrem M an metalr at a lp 'e from noted above, the purification yielded amounts of BrF varying abou U to about 340 and Separatmg bromine l from Slightly below 1 m 3 mg. From a consideration of the tafluoride from the unreacted reactants and other reaction above examples, it appears that the inclusion of CsBrF Products' generally produced crude products having greater amounts of The Process of vwherem M i an alkah metal than reactions conducted without CsBrF6 selected from the group consisting of potassium and cesium.

TABLE I Weight, Infrared; 133 C. 735 cm.- Reaction Reaction Reaction fraction hand peak Run Bl'Fs, F CsF, CsBrFs, pressure temp. time (g. crude rilativo number moles moles moles moles (p.s.l.) C.) (hrs) BrF:) height 0.108 0.137 0.177 0. 218 600 64 0.131 Medium. 0.032 0.110 0. 177 0.218 550 250 is 0. 024 Do. 0.032 0.002 0.177 0. 218 530 338 10 0.020 D0. 0.170 0.103 0. 0. 080 240 16 0. 430 Stron 0.206 2. 0. 0. 4, 300 210 12s 0. 281 Do. 0.218 2. 0. 0. 3,500 150 87 0.153 .\l((1lunl. 0.476 2. 0. 4,400 08 23.470 DQ. 0. 202 3. 0. 4. 500 122 16 0. 318 Very strong. 0.127 0. 1,100 168 0 0.272 Weak. 0.125 1. 2,050 183 44 0. 801 Strong. 0.110 2. 2.700 7 2 0. 506 Do. 0.182 2. 3,500 70 0. 475 Do. 0.197 4. 4.750 115 20 0.333 Weak. 0.207 3. 4, 650 140 64 3.105 Do. 0.185 3. 4, 550 175 16 10.010 Do. 0.181 3. 4, 300 210 68 0. 211 Medium. 0.041 0. 500 300 63 0. 343 \'cry weak. 18 0.060 0. 510 280 20 0.081 Wi-air.

TABLE I] 3. The process of claim 2 wherein the alkali metal is cesium. 4. The process of claim 2 wherein said temperature is below 60 250C.

Rem," Peak Relalive Peak 5. The process of claim 1 wherein said temperature is below me Height arr, Ion Species Height arr. 250 C- 6. A process for producing bromine heptafluoride which 93 100.0 arr. 1000 65 comprises reacting bromine pentafluoride with fluorine, in the 117 49 2 BrF,+ 113.0 presence of a compound having the formula MBrF wherein 371 M is an alkali metal, at a temperature of from about 110 C. to I55 713 B'Ffi 43,2 340 C., separating the unreacted reactants from the reaction m 0.5 arr,+ 12.0 products, purifying the reaction products by fractional 193 BYFP 0 codistillation and recovering the material eluted in the temperature range ofl 27 C. to *l 10 C.

7. The process of claim 6 wherein M is cesium. 8. The process of claim 7 wherein said temperature is below An analysis of the experimental data does not indicate any 50 c l r pr r pen y- In p ent at mosph r- 75 9. The process of claim 6 wherein said temperature is below ic pressure yielded traces of BrF,. However, higher pressures 

1. A PROCESS FOR THE PREPARATION OF BROMINE HEPTAFLUORIDE COMPRISING THE STEPS OF REACTING BROMINE PENTAFLUORIDE WITH FLUORINE IN THE PRESENCE OF A COMPOUND HAVING THE FORMULA MBRF6, WHEREIN M IS AN ALKALI METAL, AT A TEMPERATURE OF FROM ABOUT 110*C. TO ABOUT 340*C. AND SEPARATING BROMINE HEPTAFLUORIDE FROM THE UNREACTED REACTANTS AND OTHER REACTION PRODUCTS.
 2. The process of claim 1 wherein M is an alkali metal selected from the group consisting of potassium and cesium.
 3. The process of claim 2 wherein the alkali metal is cesium.
 4. The process of claim 2 wherein said temperature is below 250* C.
 5. The process of claim 1 wherein said temperature is below 250* C.
 6. A process for producing bromine heptafluoride which comprises reacting bromine pentafluoride with fluorine, in the presence of a compound having the formula MBrF6, wherein M is an alkali metal, at a temperature of from about 110* C. to 340* C., separating the unreacted reactants from the reaction products, purifying the reaction products by fractional codistillation and recovering the material eluted in the temperature range of -127* C. to -110* C.
 7. The process of claim 6 wherein M is cesium.
 8. The process of claim 7 wherein said temperature is below 250* C.
 9. The process of claim 6 wherein said temperature is below 250* C. 